Note: Descriptions are shown in the official language in which they were submitted.
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REACTOR SYSTEM FOR THE PRODUCTION OF HIGH IMPACT
POLYSTYRENE
CROSS-REFERENCE TO RELATED APPLICATIONS
100011 The present application claims priority to U.S. Provisional Application
No.
60/752,766 filed December 21, 2005 and entitled "Horizontal Boiling Plug Flow
Reactor and Reactor System for the Production of High Impact Polystyrene,"
which is
incorporated by reference. The present application relates to commonly owned
U.S.
Patent Application Serial No. 11/121,795 filed May 4, 2005 and entitled
"Reactor
Apparatus Having Reduced Back Mixing" and U.S. Patent Application Serial No.
11/384,596 [Atty. Docket No. COS-1038 (4176-00901)] filed concurrently
herewith
and entitled "Horizontal Boiling Plug Flow Reactor," both of which are
incorporated by
reference herein.
STATEMENT REGARDING FEDERALLY SPONSORED
RESEARCH OR DEVELOPMENT
[0002] Not applicable.
FIELD OF THE INVENTION
[0003] The present invention relates generally to polymer synthesis and more
particularly to the synthesis of high impact polystyrene using a combination
of
continuously stirred tank reactors and plug flow reactors.
BACKGROUND OF THE INVENTION
[00041 Elastomer-reinforced polymers of monovinylidene aromatic compounds
such as styrene, alpha-methylstyrene and ring-substituted styrene have found
widespread commercial use. For example, elastomer-reinforced styrene polymers
having discrete elastomer particles such as cross-linked rubber dispersed
throughout the
styrene polymer matrix can be useful for a range of applications including
food
packaging, office supplies, point-of-purchase signs and displays, housewares
and
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consumer goods, building insulation and cosmetics packaging. Such
elastomer-reinforced polymers are commonly referred to as high impact
polystyrene
(HIPS).
[0005] Methods for the production of polymers, such as HIPS, typically employ
polymerization using a continuous flow process. Continuous flow processes may
involve a plurality of serially arranged reaction vessels wherein the degree
of
polymerization increases from one vessel to the next. Factors such as the
arrangement
of the reaction vessels and the reaction conditions influence the
characteristics of HIPS
produced. Different grades of HIPS may have differing elastomer content and
extents
of polymerization within each reactor resulting in differing mechanical and/or
optical
properties.
[0006] Key costs for the production of HIPS are associated with the type of
continuous flow process used and the amount of elastomer utilized. Thus it
would be
desirable to develop an apparatus and methodology for the production of HIPS
having a
reduced elastomer content with enhanced mechanical properties.
BRIEF SUMMARY OF SOME OF THE PREFERRED EMBODIMENTS
[0007] Disclosed herein is a continuous process for producing high impact
polystyrene comprising feeding at least one vinyl aromatic monomer, an
elastomer, and
a free radical initiator to a first linear flow reactor to form a reaction
mixture,
polymerizing the reaction mixture in said linear flow reactor to at least the
phase
inversion point of the mixture, and feeding the reaction mixture from the
first linear
flow reactor to a second reactor for post-inversion polymerization of the
mixture.
[0008] Further disclosed herein is a method of producing a elastomer-
reinforced
polymer comprising inverting a reaction mixture comprising at least one vinyl
aromatic
monomer, an elastomer, and a free radical initiator in a plug flow reactor.
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[0009] Further disclosed herein is a high impact polystyrene reactor system,
comprising a linear flow reactor having an inlet for receiving at least one
vinyl aromatic
monomer, an elastomer, and a free radical initiator and an outlet for
conveying a reactor
effluent, and a continuously stured tank reactor having an inlet in fluid
communication
with the linear flow reactor outlet and receiving the effluent from the linear
flow
reactor.
[0010] The foregoing has outlined rather broadly the features and technical
advantages of the present invention in order that the detailed description of
the
invention that follows may be better understood. Additional features and
advantages of
the invention will be described hereinafter that form the subject of the
claims of the
invention. It should be appreciated by those skilled in the art that the
conception and
the specific embodiments disclosed may be readily utilized as a basis for
modifying or
designing other structures for carrying out the same purposes of the present
invention.
It should also be realized by those slalled in the art that such equivalent
constructions
do not depart from the spirit and scope of the invention as set forth in the
appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
100111 For a detailed description of the preferred embodiments of the
invention,
reference will now be made to the accompanying drawings in which:
[0012] Figure 1 is a schematic representation of an apparatus for HIPS
production.
[0013] Figure 2a is a diagram of a linear flow reactor.
[0014] Figure 2b is a cross-sectional view of an internal reactor cooling
component.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIlVIENTS
[0015] A schematic representation of a reactor system 100 for the continuous
production of a elastomer-reinforced polymer is given in Figure 1. In an
embodiment,
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reactor system 100 is useful for a continuous HIPS production process.
Referring to
Figure 1, a reaction mixture comprising styrene, an elastomer such as
polybutadiene
rubber, and a free radical initiator may be fed to a polymerization reactor
10, through a
feed line generally indicated at 5. Alternatively, the reaction mixture
comprises styrene,
an elastomer such as polybutadiene rubber, a chain transfer agent and
additional
components such as those known in the art for the production of HIPS.
Alternatively,
the reaction mixture comprises styrene, an elastomer such as polybutadiene
rubber, a
combination of a free radical initiator and chain transfer agent and
additional
components such as those known in the art for the production of HIPS. The
nature and
amount of free radical initiator, chain transfer agent and additional
components for the
production of HIPS may be included as known to one of ordinary skill in the
art. Such
a feed line may allow for introduction of the reaction mixture through the
bottom of the
reactor, as shown in Figure 1, alteinatively such a feed line may allow for
introduction
of the reaction mixture through the top of the reactor, altematively through
any position
along the reactor vessel that is compatible with the reaction mixture and the
reactor
equipment.
[0016] In an embodiment, a reaction mixture for introduction to the PFR may
comprise from about 75% to about 99% styrene, from about 1% to about 15%
polybutadiene, from about 0.001% to about 0.2% free radical initiator and
additional
components as needed to impart the desired physical properties. The percent
values
given are percentages by weight of the total composition. As used herein the
term
styrene includes a variety of substituted styrenes (e.g., alpha-methyl
styrene), ring-
substituted styrenes such as p-methylstyrene as well as unsubstituted
styrenes.
[0017] The polymerization reactor 10 may be a linear-flow reactor, such as a
plug
flow reactor (PFR) shown in more detail in Figure 2a. In an embodiment, the
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polymerization reactor 10 is arranged vertically as shown in Figure 1. In an
altemative
embodiment, the polymerization reactor 10 is arranged horizontally in the
apparatus.
[0018] Polymerization reactor 10 may be operated under conditions that allow
the
polymerization reaction to proceed to at least the point of phase inversion
before the
reaction mixture is introduced to any additional polymerization reactors. As
such
polymerization reactor 10 is termed a plug flow inversion reactor (PFIR).
Stated
alternatively, the reactants in polymerization reactor 10 undergo phase
inversion prior
to exiting the reactor, referred to here after as PFIR 10.
[0019] Phase inversion refers to a morphological transformation that occurs
during
the preparation of HIPS. In an embodiment, HIPS preparation involves the
dissolution
of polybutadiene rubber in styrene that is subsequently polymerized. During
polymerization, a phase separation based on the immiscibility of polystyrene
and
polybutadiene occurs in two stages. Initially, a mixture of styrene and
polybutadiene
forms the major or continuous phase with a mixture of polystyrene and styrene
dispersed therein. However, as the reaction of styrene into polystyrene
progresses and
the amount of polystyrene increases, a morphological transformation or phase
inversion
occurs such that the polystyrene/styrene mixture forms the continuous phase.
This
phase inversion leads to the formation of complex rubbery particles in which
the rubber
exists in the form of inembranes surrounding occluded domains of polystyrene.
[0020] Referring again to Figure 1, the PFIR 10 may contain agitators 14
driven by
a motor 12. Such agitators may promote radial dispersion of the reactants but
are not
intended to provide axial mixing so as to minimize backmixing in the reactor.
A
similar linear flow reactor design has been disclosed in U.S. Patent
Application Serial
No. 11/384,596 [Atty. Docket No. COS-1038 (4176-00901)] filed concurrently
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herewith entitled "Horizontal Boiling Plug Flow Reactor," which is
incorporated by
reference herein.
[00211 As polymerization reactions are highly exothermic, means are required
to
control the temperature in the reaction vessel as the polymerization proceeds.
In an
embodiment, shown in Figure 2a, heat is removed through intemal cooling coils
in the
PFIR 10. Figure 2b is a cross-sectional view of an internal cooling coil taken
along line
A-A in Figure 2a. In Figure 2a, arrow I indicates the reaction process flow in
the PFIR,
which is an upward flow of reactants in a vertical reactor, as shown in Figure
1, but
with the further understanding that reactor orientation and flow direction can
vary as
discussed previously. The PFIR 10 may be a dual wall reactor having an inner
wall 101
and outer wall 102. Outer flow channels 105 are disposed between the inner and
outer
walls such that a coolant, such as thenmal oil, may be introduced to the PFIR
10 at inlet
ports to the intemal cooling coils shown as 3, 7 and 9. The coolant may then
circulate
throughout the reactor cooling coils and exit the system through the outlet
ports 11, 13
and 17. Inner flow channels 106 of the cooling coil may traverse the breadth
of the
reactor and connect the outer flow channels 105. Coolant flowing through the
internal
cooling coils functions as a heat exchanger that enables the removal of excess
heat from
the polymerization reaction.
[0022] Referring again to Figure 1, the apparatus may further comprise an
additional polymerization reactor, 20, located downstream of polymerization
reactor 10.
Output from polymerization reactor 10 may be fed to polymerization reactor 20
via line
15. In an embodiment, polymerization reactor 20 is a continuously stirred tank
reactor
(CSTR) having an agitator 18 driven by a motor 16.
[0023] The polymerization of styrene to polystyrene may continue with the
output
from polymerization reactor 20 being fed to additional polymerization
reactors, 30 and
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40, via lines 25 and 55, respectively. In an embodiment, reactors 30 and 40
may be
linear-flow reactors, such as a plug flow reactors, that may also be equipped
with
agitators 22 and 26 driven by motors 24 and 28, respectively. In the
embodiment
shown in Figure 1, the two linear flow reactors 30 and 40 are horizontally
oriented and
serially connected to polymerization reactor 20 with increased polymerization
occurring
in each subsequent reactor. In an embodiment, reactor system 100 may comprise
any
number of additional reactors downstream of reactor 20 (e.g., CSTR 20) as
desired by
the user. The number, orientation (e.g., horizontal or vertical), and
connectivity (e.g.,
serial or parallel) of the linear flow reactors may be determined by one
skilled in the art
based on requirements such as production capacity required or extent of
product
conversion desired. The resultant HIPS polymer and any other remaining
compounds
may be removed from the final reactor, e.g., reactor 40, via line 75, and
thereafter the
HIPS polymer may be recovered and optionally further processed, such as
pelletized.
100241 In an embodiment, unreacted styrene monomer and other volatile residual
components may exit any of the reactors or downstream processing equipment
(not
shown) as a recycle stream. In general, a recycle stream may be recovered from
any
downstream reactor and retumed to any one or more suitable upstream reactors.
In the
embodiment shown in Figure 1, a recycle stream exiting a separation device
downstream of reactor 40 may be returned upstream to polymerization reactor 20
at line
45. Alternatively, as shown in Figure 1, a recycle stream exiting
polymerization reactor
may be returned upstream to polymerization reactor 20 via line 35.
Altematively, as
shown in Figure 1, a recycle stream exiting polymerization reactor 40 may be
returned
upstream to polyrnerization reactor 30 via line 65. In an embodiment, the
recycle
stream undergoes recycle treatment designed to increase the purity of the
recycle
25 components, e.g., styrene, before being reintroduced to a reactor. Methods,
conditions
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and apparatuses for carrying out recycle treatments are known to one of
ordinary skill in
the art.
100251 In an embodiment, the HIPS produced by the disclosed apparatus and
process configuration has a reduced rubber (e.g., polybutadiene rubber)
content while
having similar or enhanced mechanical and/or optical properties when compared
to a
HIPS produced using a conventional process configuration and apparatus.
Conventional process configurations and apparatuses for the production of HIPS
are
known to one of ordinary skill in the art. For example, a conventional process
configuration and apparatus may employ two CSTRs (i.e., reactors 10 and 20 in
Figure
1) as the first and second polymerization reactors prior to feeding the
reaction mixture
to some number of linear flow reactors (i.e., reactors 30 and 40 in Figure 1).
In an
embodiment, the rubber content of the HIPS produced by the disclosed apparatus
and
process configuration is reduced by equal to or greater than about 5%,
alternatively
about 10%, but has similar or enhanced mechanical and/or optical properties
when
compared to HIPS produced by a conventional process configuration and
apparatus.
Hereafter, HIPS having a reduced rubber content will be denoted rHIPS while
HIPS
produced using a conventional process configuration and apparatus will be
denoted
nHIPS.
[0026] The rHIPS may display an impact strength similar to or improved in
comparison to nHIPS when using standard tests of impact strength such as the
Izod
impact and falling dart test. Izod impact is defined as the kinetic energy
needed to
initiate a fracture in a specimen and continue the fracture until the specimen
is broken.
Tests of the Izod impact strength determine the resistance of a polymer sample
to
breakage by flexural shock as indicated by the energy expended from a pendulum
type
hammer in breaking a standard specimen in a single blow. The specimen is
notched
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which serves to concentrate the stress and promotes a brittle rather than
ductile fracture.
Specifically, the Izod Impact test measures the amount of energy lost by the
pendulum
during the breakage of the test specimen. The energy lost by the pendulum is
the sum
of the energies required to initiate sample fracture, to propagate the
fracture across the
specimen, and any other energy loss associated with the measurement system
(e.g.,
friction in the pendulum bearing, pendulum arm vibration, sample toss energy).
In an
embodiment, the rHIPS of this disclosure has an Izod impact strength of 1.5
ft.lb/inch to
3.5 ft.lb/inch, alternatively, 2 ft.lb/inch to 3 ft.lb/inch, altematively 2.8
ft.lb/inch.
[0027] The falling dart impact test is also a standard test of polymer impact
resistance. Specifically, it is the energy required to rupture a film. The
test is
conducted by determining the weight of a dart dropped from a height of 26
inches that
causes 50% of the samples to break. In an embodiment, the rHIPS of this
disclosure
has a falling dart impact strength of 40g to 200g, alternatively, of greater
than 100g.
[0028] In an embodiment, the rHIPS may also display ductile properties such as
bend or elongation similar to or improved in comparison to that of nHIPS.
Ductile
properties such as bend or elongation indicate the ability of a material to
deform
elastically until a fracture or break point. Specifically, the elongation of a
polymer
sample is typically given as the percent elongation, which refers to the
length the
polymer sample is after it is stretched (L), divided by the original length of
the sample
(LO), and then multiplied by 100. The bend of a polymer sample refers to the
number of
times a specimen constructed of the polymeric material may be bent before it
fractures.
In an embodiment the rHIPS of this disclosure has a bend of 10 to 150,
alternatively, 20
to 90, alternatively, 70 and an elongation of 2% to 80%, alternatively, 40% to
70%,
alternatively 70%.
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EXAMPLES
[0029] The invention having been generally described, the following examples
are
given as particular embodiments of the invention and to demonstrate the
practice and
advantages thereof. It is understood that the examples are given by way of
illustration
and are not intended to liniit the specification of the claims in any manner.
EXAMPLE 1
[0030] A high impact polystyrene was produced using a polymerization reactor
configuration as disclosed herein and depicted in Figure 1. The mechanical and
optical
properties of this high impact polystyrene material produced using the
disclosed reactor
system of Figure 1 was compared to that of high impact polystyrene material
produced
by a standard apparatus and process configuration having the first two
reactors as
CSTRs (e.g., reactor 10 replaced with a CSTR in Figure 1). The high impact
polystyrene material produced by a standard apparatus and process
configuration has
typical properties as set forth in Table 1A and is a high impact strength
resin that is
suitable for applications such as custom sheet extrusion or thermoforniing,
printing
surfaces and packaging.
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Table 1A
ASTM Test Typical Value
Melt Flow
Flow, gm/10 min., 200/5.0 D-1238 3.0
Impact Properties
Falling Dart, in-lb D-3029 125
Izod, ft-lbs/in, notched D-256 2.2
Tensile Properties
Strength, psi D-638 3,500
Modulus, psi 10 D-638 3
Elon ation, % D-638 55
Flexural Properties
Strength, psi D-790 6,900
Modulus, psi 10 D-790 3.2
Thermal Properties
Heat Distortion, F Annealed D-648 201
Vicat Softenin , F D-1525 210
Optical Properties
Gloss, 60 D-523 70
[0031] Four experimental trials were conducted using variations in reagents
and/or.
process configurations of the reactor system of Figure 1. In Trial 1, a
standard HIPS-
type reactant feed, given in Table 1B, was introduced to the PFIR and reacted
to
produce HIPS. TAKTENE 380/550 are butadiene rubbers commercially available
from
Lanxess.
[0032] Trial 2 was similar to Trial lwith the exception that a different free
radical
initiator was used and the rubber type was all TAKTENE 550. Low rubber
conditions,
6% and 5.5%, were also run in this trial. Trial 3 had similar reaction
conditions to Trial
1 however the process was configured such that recycling of unreacted styrene
and
volatile monomers occurred at different reactors than in previous trials.
Trial 4 was
similar to Trial 3 however DIENE 70 rubber manufactured by Firestone was used
in
place of TAKTENE rubber and a low rubber condition, 5.5%, was also run in this
trial.
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Table 1B
Trial Rubber Type Rubber
1 50 /a TAKTENE 380 7
50% TAKTENE 550
[0033] As shown in Table 2, the mechanical and optical properties of HIPS
produced
in Trials 1 through 4 were compared to the standard HIPS product (produced via
the
standard reactor configuration described previously) as determined in
accordance with
the appropriate ASTM method given in parentheses.
Table 2
Physical Property Standard 825E Trial 1 Tria12 Tria13 Tria14
Melt Flow Index 2.8 2.6 3.1 3.5 2.9
(ASTM D 1238)
Gloss, 60 (%) 52 ND** 50 34 57
ASTM D 523)
Izod (ft.-lb/in) 2.6 3.5 3.0 2.9 2.8
(ASTM D 256)
Falling Dart (in-lb) 67 78 80 66 90
(ASTM D 3029)
Bends 20 54 58 36 36
% Elon ation 49 50 58 63 54
RPS* nricron 5.4 3.9 3.7 4.7 3.4
* RPS= rubber par6cle size as detercnined by laser light scattering apparatus
ND= not detennined
[0034] The results demonstrate that the HIPS produced in Trials I through 4
have
better impact properties, as reflected in an improved Izod and falling dart
impact
strength than the standard HIPS. Furthermore, the HIPS produced in Trials I
through 4
had improved ductile properties such as bend and elongation when compared to
the
standard HIPS.
100351 Table 3 compares the physical properties of HIPS produced in Trials 2
and 4
under low rubber conditions. Samples from Tria12 were run using either 5.5% or
6%
rubber as indicated. The physical properties were determined in accordance
with the
appropriate ASTM method given in parentheses.
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Table 3
Physical Property Standard 825E Trial 2 Tria12 Trial 4
% Rubber 7 5.5 6 5.5
Melt Flow Index 2.8 3.4 3.7 3.2
(ASTM D 1238)
Gloss,60 (%) 52 57 54 56
(ASTM D 523)
Izod (ft.-lb/in) 2.6 2.1 2.4 2.2
(ASTM D 256)
Falling dart (in-lb) 67 72 70 85
(ASTM D 3029)
Bends 20 41 50 31
% Elon ation 49 56 57 56
RPS micron 5.4 4.7 5.0 3.9
[0036] The results demonstrate that the 6% rubber HIPS in Tria12 comes the
closest
to matching the Izod of the standard HIPS. In this case, Izod was only
slightly worse
and would still be considered as acceptable. However, even at 5.5% rubber, all
impact
and ductile properties beside Izod for the HIPS produced in Trials 2 and 4 are
superior
to the standard HIPS.
100371 While preferred embodiments of the invention have been shown and
described, modifications thereof can be made by one skilled in the art without
departing
from the spirit and teachings of the invention. The embodiments described
herein are
exemplary only, and are not intended to be limiting. Many variations and
modifications
of the invention disclosed herein are possible and are within the scope of the
invention.
Where numerical ranges or limitations are expressly stated, such express
ranges or
limitations should be understood to include iterative ranges or limitations of
like
magnitude falling within the expressly stated ranges or limitations (e.g.,
from about 1
to about 10 includes, 2, 3, 4, etc.; greater than 0.10 includes 0.11, 0.12,
0.13, etc.).
Use of the term "optionally" with respect to any element of a claim is
intended to mean
that the subject element is required, or alternatively, is not required. Both
alternatives
are intended to be within the scope of the claim. Use of broader terms such as
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comprises, includes, having, etc. should be understood to provide support for
nar'rower
ternis such as consisting of, consisting essentially of, comprised
substantially of, etc.
[0038] Accordingly, the scope of protection is not limited by the description
set out
above but is only limited by the claims which follow, that scope including all
equivalents of the subject matter of the claims. Each and every claim is
incorporated
into the specification as an embodiment of the present invention. Thus, the
claims are a
further description and are an addition to the preferred embodiments of the
present
invention. The discussion of a reference herein is not an admission that it is
prior art to
the present invention, especially any reference that may have a publication
date after the
priority date of this application. The disclosures of all patents, patent
applications, and
publications cited herein are hereby incorporated by reference, to the extent
that they
provide exemplary, procedural or other details supplementary to those set
forth herein.
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